1. Field of the Invention
The present invention relates to a hologram and, particularly, it relates to the adhesion between a hologram element and its cover film.
2. Description of the Related Art
Gelatin dichromate and photopolymers have conventionally been used as photosensitive materials for hologram elements.
Hologram elements made of gelatin dichromate have high diffraction efficiency and produce bright reproduction images. They also have a problem of susceptibility to moisture absorption which leads to changes in the hologram properties.
Also, while hologram elements made of photopolymers are moisture-resistant, they also have problems of poor solvent resistance and chemical resistance. For this reason, they have had poor quality reliability in high temperature environments and high temperature, high moisture environments.
Thus, according to the prior art it has been necessary to protect hologram elements from their environments.
In conventional use of hologram elements made of gelatin dichromate, the first means of protecting the hologram element was to cover the surface of the hologram element with a glass plate, and the second means was to produce the hologram element in an ultra-low humidity environment (dew point of -20.degree. C. or below).
Also, in conventional use of hologram elements made of photopolymers, the first means has been to provide an inorganic layer on the plate surface, to prevent migration of water-soluble solvents into the hologram element caused by penetration of moisture passing through the plate to the hologram element from the plate surface (Japanese Unexamined Patent Publication No. 64-40882).
A second means has also been proposed whereby the hologram element is attached to a plate using an adhesive (Japanese Unexamined Patent Publication No. 5-203812). Such optical adhesives employ special materials to maintain the properties of the hologram element. As optical adhesives of such type there have been proposed, for example, adhesives containing no plasticizers (Japanese Unexamined Patent Publication No. 6-56484) and adhesives comprising polyfunctional acrylates and polymerization initiators (Japanese Unexamined Patent Publication No. 3-157684).
However, the following problems have been associated with the conventional means for protecting hologram elements.
First, with the above-mentioned first means for using hologram elements made of gelatin dichromate, the use of glass plates has been an obstacle to weight reduction of the hologram elements, while their possible uses have been limited because of the reduced degree of design freedom, since the shape must be flat.
With the second means, production under ultra-low humidity conditions has resulted in poor manageability.
The first means for using hologram elements made of photopolymers is limited to cases where the hologram element is a polyvinyl carbazole system, because when other photopolymers are used certain problems occur such as changes in the properties of the hologram caused by solvents or chemicals, discoloration of the hologram element, and inferior durability.
Also, with the second means, there has been a problem in that air bubbles become included in the adhesive during adhesion of the hologram element, thus altering the optical properties of the hologram element. Here, addition of an antifoaming agent to the adhesive has been considered as an option. However, although this allows absorption of relatively small air bubbles, the larger air bubbles which are produced during attachment cannot be absorbed.
In addition, after application of the adhesive to the hologram element and attachment to the plate, the adhesive often bleeds out from between them. For this reason, a step has been necessary for wiping off the adhesive which bleeds out.
Also, when the positions of the hologram element and the plate become shifted and they are then attached with the adhesive, the hologram element and plate become fixed with the adhesive, making it impossible to reposition them afterwards.